It cites one such group, Anonymous Caucasus, which claims "the Sochi games infrastructure was built on the graves of 1 million innocent Caucasians who were murdered by the Russians in 1864," according to the Homeland Security bulletin.

Anyone attending the games "will likely" have their communications monitored, the bulletin warned, and cyber criminals may use the games as a way to spread malware or phishing scams to sports audiences around the world.

The alert came as a denial of service attack was launched on Internet sites related to the games, knocking Russia's national games committee offline Wednesday.

Anonymous Caucasus announced on Twitter that it had called on its members to take action against Sochi-related sites, using the hashtags #paybackforsochi and #opsochi.

On a related Twitter account, hackers posted a message to the Russian government and media: "Your cruelty to animals is enough to take down Your entire country, and we plan on it. Welcome to WAR Russia."

However, the official Twitter feed of Anonymous Caucasus said the action was a protest at the 19th Century deportation of thousands of native Circassians from the region.